Characterization of α-isosaccharinic acid: Lactone and carboxylic conformations

Article abstract of DOI:10.1023/B:JOSL.0000037771.45926.f2

The equilibrium concentrations of α-D-isosaccharino-1,4-lactone (ISA_L) and the carboxylic α-D-isosaccharinic acid (HISA) conformations as a function of pH, as well as the rate of the structural rearrangements between the species, have been esta

Full text of DOI:10.1023/B:JOSL.0000037771.45926.f2

Journal of Solution Chemistry, Vol. 33, No. 5, May 2004 (ꢀC 2004)
Characterization of α-Isosaccharinic Acid:
Lactone and Carboxylic Conformations
S. Ekberg,1 C. Ekberg, and Y. Albinsson
,∗
1
1
Received January 20, 2004; revised April 26, 2004
The equilibrium concentrations of α-D-isosaccharino-1,4-lactone (ISAL) and the car-
boxylic α-D-isosaccharinic acid (HISA) conformations as a function of pH, as well
as the rate of the structural rearrangements between the species, have been estab-
◦
lished in a 0.1 M NaClO4 matrix at 23 C using ion chromatographic analytical
+
H
techniques. The equilibrium constant for the reaction HISA ←→ ISAL + H2O was
determined to be K1 = 6.88 ± 0.64 by curve fitting the experimental equilibrium data.
The transformation rate from HISA to ISAL exhibits first-order behavior with a half-
life of t /2
1
= 122.2 ± 0.6 min. On the basis of kinetic data, the equilibrium constant
was determined to be 6.60 ± 0.07 with the forward and backward rate constants be-
+
−3
−1
−
−4
−1
ing k = (4.85 ± 0.03) × 10 min and k = (7.35 ± 0.04) × 10 min , respec-
tively, in agreement with the curve-fitted equilibrium constant. The transformation from
−
ISAL to ISA is more rapid and is complete in less than 8 min.
KEY WORDS: α-D-isosaccharinic acid; α-D-isosaccharinate; α-D-isosaccharino-1,4-
lactone; structural rearrangements; equilibrium constants; acid dissociation constants;
rate constants; ion chromatography.
1. INTRODUCTION
The Swedish repository for long-lived, low to intermediate level waste will
contain cellulose-based items from research facilities, laboratories, hospitals, and
nuclear power plants. The repository is planned to be a mainly concrete and
steel structure called SFL3-5.( Alkaline conditions in the repository, originating
1)
from dissolution of concrete, will lead to degradation of cellulose and subsequent
formation of isosaccharinic acid, for example.(
2–4)
This carboxylic acid is a strong
complexing agent for tetravalent actinides, as well as for trivalent lanthanides
and actinides, and affects the solubilities of these elements and their interactions
1Nuclear Chemistry, Department of Materials and Surface Chemistry, Chalmers University of Tech-
nology, SE-412 96 G o¨ teborg, Sweden; e-mail: stefane@chem.chalmers.se.
4
65
0
095-9782/04/0500-0465/0 ꢀC 2004 Springer Science+Business Media, Inc.

466
Ekberg, Ekberg, and Albinsson
Fig. 1. α-D-Isosaccharino-1,4-lactone, C6H10O5, andα-D-isosaccharinic
acid, C6H12O6.
with the cement of the repository.(5–8) As such it is of great interest for long-term
assessments.
Two diastereomers, α- and β-D-isosaccharinic acid are produced in approxi-
mately equal amounts by the alkaline degradation of cellulose. Each diastereomer
in turn has two corresponding enantiomeric lactone conformations.(
3,4,9)
The con-
formations are pH-dependent with the isosaccharinate form dominating at higher
pH, and the lactone form at lower pH. The α-D-isosaccharino-1,4-lactone and
carboxylic α-D-isosaccharinic acid studied here are both shown in Fig. 1. How-
ever, little information is available regarding the transformation between these
two forms. Previous studies have indicated that the transformation occurs at ap-
proximately pH 8.( No detailed studies regarding the transformation from the
lactone to the carboxylic form, and vice versa, have been performed and only data
pertaining to analogues are available.(
10)
11)
The objective of this study is to determine the equilibrium concentrations
of the two forms at different pH, and the rate of transformation from the lactone to
the carboxylic form as well as from the carboxylic to the lactone form. This infor-
mation is of relevance in complexation studies and in the general determination
of the chemical properties of the two forms.
2. THEORY
In the following text α-D-isosaccharinic acid will be abbreviated HISA and
α-D-isosaccharino-1,4-lactone, ISAL. The protonation of the base form of HISA
can be written as
−
+
ISA + H ←→ HISA
(1)
(2)
The stability constant for reaction (1) is expressed as
[
HISA]
β∗ =
−
+
[
ISA ][H ]

Characterization of α-Isosaccharinic Acid: Lactone and Carboxylic Conformations
467
and has been determined under varying experimental conditions.(12–17) The asterisk
indicates that the activity coefficients are included in the value of the stability
∗
constant β . The acid-catalyzed transformation from HISA to ISAL is expressed
in the following way:
+
H
HISA ←→ ISAL + H2O
(3a)
(3b)
(3c)
+
k
HISA −→ ISAL + H2O
−
k
HISA ←− ISAL + H2O
+
−
where k and k are the forward and backward reaction rates.
The equilibrium constant for reaction (3a) is expressed as
[
ISAL]
K1 =
(4a)
(4b)
[HISA]
or using the reaction rates of reactions (3b) and (3c),
+
k
K1 =
−
k
The nature of the dominating species of the two different conformations of isosac-
charinic acid is dependent on pH with the lactone form dominating at lower pH
and the carboxylic form at higher pH. The pH dependence can be shown utilizing
Eqs. (2) and (4a), and can be expressed as
[
ISAL]
∗
+
=
K1β [H ]
(5)
−
[ISA ]
The mass balance for isosaccharinic acid and the lactone form can be expressed
as
−
[
ISA]tot = [ISA ] + [HISA] + [ISAL]
(6)
First-order reactions have the form
dC
dt
−
= λC
(7)
where C is the concentration of the species of interest and λ is a constant. By
integrating Eq. (7), an expression for reaction (3a) can be reached using Eq. (6),
ꢀ
ꢁ
−
[
ISA ] + [HISA]
−
ln
= λt
(8)
[ISA]tot
−
Given that the ion chromatograph detects the sum of ISA and HISA, as further
−
explained in Section 3.3.3, it is practical to introduce the equality α = ([ISA ] +
−
[
HISA])/[ISA]tot. At equilibrium this equality becomes αeq = ([ISA ]eq +

468
Ekberg, Ekberg, and Albinsson
[
HISA]eq)/[ISA]tot with [ISA]eq and [HISA]eq being the equilibrium concentra-
−
tions of ISA and HISA. These equalities applied to Eq. (8) result in,
−
ln (α) = λt
(9a)
and can be rearranged in terms of the equilibrium concentrations,
ꢀ
ꢁ
α − αeq
−
ln
= − ln(α)corr = λt
(9b)
1
− αeq
Expressing [ISAL] using reactions (3b) and (3c), the following equation can also
be derived:
d[ISAL]
+
−
=
(k + k )([ISAL]eq − [ISAL])
(10)
dt
where [ISAL]eq is the equilibrium concentration of ISAL. Its integration yields
ꢀ
ꢁ
[
ISAL]eq − [ISAL]
ISAL]eq
+
−
−
ln
= (k + k )t
(11)
[
−
Using Eq. (6), realizing that the concentration of ISA under the experimental
conditions of the transformation rate study is negligible, leads to,
−
[
ISAL]eq = [ISA]tot − [ISA ]eq − [HISA]eq
(12)
and,
−
[
ISAL] = [ISA]tot − [ISA ] − [HISA]
(13)
Eq. (11) now can be expressed as
ꢀ
ꢁ
α − αeq
+
−
−
ln
= (k + k )t
(14)
(15)
(16)
1
− αeq
A comparison of Eqs. (9b) and (14) yields obvious the relation
λ = k + k−
+
The half-life, t , is given through
1
/
2
ln (2)
t
1
=
/
2
λ
To assess the activity coefficients at ionic strengths up to Im < 3.5 M, the
specific ion interaction theory, SIT, can be used(
18–21)
ꢂ
2
log γj = −z D +
ε( j,k,I )mk
(17)
j
m
k
where γj is the activity coefficient in the molal scale for an ion j, z is the charge
of the ion j and ε( j,k,I ) is the specific ion interaction parameter for species j and
m

Characterization of α-Isosaccharinic Acid: Lactone and Carboxylic Conformations
469
(18)
(19)
k at the ionic strength Im. The ionic strength is expressed as
ꢂ
1
Im =
mi z2
i
2
i
where m is the molality of the solution. The Debye–H u¨ ckel term is written
√
A
Im
D =
√
1
+ Baj Im
where A and B are temperature- and pressure-dependent constants and aj is a
parameter related to the size of the hydrated ion j.
3. EXPERIMENTAL
3.1. Chemicals
Milli-Q water, which was double distilled and ion exchanged water, was
consistently used in the preparation of all solutions. Isosaccharinic solutions were
prepared from Alfa-Aesar calcium α-D-isosaccharinate (Ca(ISA)2), 98% purity.
Sigma Dowex-50W, 8% cross-linkage, 76689-17-3 ion-exchange resin was em-
ployed in the removal of calcium. A 0.1 M NaClO4 solution was prepared from
Merck sodium perchlorate monohydrate, pro analysi. Solutions of 1.0 M NaOH,
0
.1 M NaOH, 0.1 M HClO4, and 1 M HNO3, were prepared with Riedel-de Ha e¨ n
Titrisol. Standard buffers used in the pH-electrode calibration were pH 7.000,
10.012, and 12.45 from Radiometer Analytical and pH 4.01 from Mettler-Toledo.
3.2. Equipment
Centrifugation was performed with a Beckman Model J2-21 centrifuge. The
pH was measured using a Radiometer Copenhagen PHC3006-9 saturated NaCl
combination electrode connected to a Radiometer Copenhagen ABU91 Autobu-
−
rette. Ion chromatographic analyses of ISA were performed with a conductivity
detector coupled to a Dionex DX-100 ion chromatograph, configured with an Ion-
Pac ICE-AS6 organic column and a AMMS-ICE II suppressor, using a 0.4 mM
heptafluorobutyric acid eluent and 5 mM tetrabutylammonium hydroxide regener-
ant. Ion chromatographic analyses of calcium were performed with a conductivity
detector coupled to a Dionex DX-100 ion chromatograph, configured with an
IonPac CS12A cation column with an attached IonPac CG12A guard column and
a CSRS-ULTRA self-regenerating suppressor, using a 20 mM methanesulfonic
acid eluent. Spectrophotometric analyses were performed using a Perkin Elmer
Lambda 19 UV/VIS/NIR spectrophotometer. Weight measurements were done
with a Mettler AE260 DeltaRange balance.

470
Ekberg, Ekberg, and Albinsson
3.3. Method
3.3.1. Preparation of [ISA]tot Solution
A [ISA]tot batch solution was prepared using calcium α-D-isosaccharinate,
0.1 M NaClO4, and 0.1 M HClO4. The solution contained some undissolved
calcium α-D-isosaccharinate and was centrifuged for 30 min at 15,000 g. By anal-
ysis of the calcium concentration and stoichiometrical calculations, the resulting
[ISA]tot solution was determined to be 58.8 mM. Following the centrifugation, the
supernatant was withdrawn and added to a beaker with acidified and washed ion-
exchange resin, and shaken for a few minutes. The volume ratio of the supernatant
to the resin was approximately 2:1. The procedure was performed twice, resulting
in the lowering of the calcium concentration from 29.4 to 0.026 mM, equivalent to
a 99.9% calcium reduction. The [ISA]tot concentration of the resin-treated solution
+
was 44.7 mM, corresponding to 76% recovery. The −log[H ] of this solution was
determined to be 1.05 by means of titration.
◦
3
.3.2. Studies of Transformation Rates at 23 C
−
To determine the rate at which ISAL is transformed to ISA , an excess amount
of 0.1 M NaOH was added to the 44.7 mM [ISA]tot solution, creating a 5.7 mM
ISA]tot solution of about pH 13. Analysis by ion chromatography commenced
immediately upon addition of NaOH with one analysis being performed every
[
10 min for 330 min.
To determine the transformation rate of HISA to ISAL, an excess amount
of 0.1 M HClO4 and minor additions of 1 M HNO3 were added to a 39.8 mM
alkaline [ISA]tot solution, creating a 5.0 mM [ISA]tot solution of about pH 1.
Analysis by ion chromatography commenced immediately upon addition of acid
with one analysis being performed every 10 min for the first 250 min, after which
analyses were performed every 20 min except for two intervals of 208 and 414 min.
Analysis was completed after 1290 min.
3.3.3. Studies of Equilibrium
Using the resin-treated [ISA]tot solution, 13 different solutions with 5–6 mM
[ISA]tot at varying pH values were prepared through the addition of 0.1 M NaOH,
keeping the ionic strength of the solution at 0.1 M. In the pH region of 7–9
where significant pH fluctuations were observed because of the differences in
transformation rates, automatic end-point titrations were carried out on three
of the samples, specifically to reach pH 7, 8, and 9. Multipoint calibrations of
the electrode using standard buffers of pH 4.01, 7.00, 10.012, and 12.45 were
performed prior to measurement. Analyses of the solutions were performed using
−
ion chromatography in a configuration to selectively detect ISA , which appeared
−
−
as a symmetrical peak at 6.05 min. The detected ISA is the sum of ISA and

Characterization of α-Isosaccharinic Acid: Lactone and Carboxylic Conformations
471
HISA in the analyzed sample, as HISA is deprotonated in the suppressor. The
−
equilibrium experiments were terminated when no further change in pH and ISA
concentration was observed. In addition, the equilibrated solutions were analyzed
using spectrophotometry in the interval 180 < λ < 500 nm. Blanks consisting of
solutions with pH and ionic strength equivalent to that of the different solutions
were utilized in this analysis.
4. RESULTS AND DISCUSSION
The results from the transformation rate studies indicate a considerable rate
−
difference depending on whether the transformation is from ISAL to ISA or
from HISA to ISAL. The transformation from HISA to ISAL exhibits first-order
behavior, see Figs. 2 and 3, and Table A.I.
The leveling out that can be observed in Fig. 2 is due to equilibrium being
reached. A solution with a pH lower than 3.5, equivalent to that of the 0.4 mM
heptafluorobutyric acid eluent, may possibly experience some transformation from
the lactone to the carboxylate form as the sample traverses the system, resulting
in a slightly higher detector signal. Such transformations have not been observed.
Linearization of Eq. (9b) allows the determination of λ, see Fig. 3.
Under the experimental conditions used here, i.e., an excess acid environment,
we can neglect the effects of reaction (1) on the transformation rate from HISA to
Fig. 2. α as a function of time after addition of excess acid to
alkaline [ISA]tot solution is shown.

472
Ekberg, Ekberg, and Albinsson
Fig. 3. ln(α)corr, as expressed in Eq. (9b), is shown as a function of
time after addition of excess acid to an alkaline [ISA]tot solution. λ
is determined to be −(5.67 ± 0.03) × 10−
3
.
ISAL as expressed in reaction (3a). Thus, Eq. (4a) can be written as
[
ISAL]eq
HISA] − [ISAL]eq
Analysis of the transformation data can now be carried out using Eqs. (15) and (20)
K1 =
(20)
[
−
3
together with λ = −(5.67 ± 0.03) × 10 , enabling us to determine the equilib-
rium constant K1, as well as the forward and backward rate constants of reactions
(
3b) and (3c). The equilibrium constant is determined to be 6.60 ± 0.07 with the
−3
−1
−
−4
reaction rates being k = (4.85 ± 3) × 10 min and k = (7.35 ± 0.4) × 10
−1
min , respectively. The transformation from HISA to ISAL has a half life of
t
1
= (122.2 ± 0.6) min. Keeping in mind that the ion chromatograph detects the
/
2
−
sum of ISA and HISA, the transformation can be described by,
−tln (2)
t
−
1/
2
[
ISA ] + [HISA] = ([ISA]tot − [ISA]eq)e
+ [ISA]eq
(21)
+
with [H ] = 0.1 M, [ISA]tot = 5.04 mM, [ISA]eq = 0.66 mM, and [HISA] ꢁ
−
[
ISA ], Eq. (21) can be expressed as,
−tln (2)
t
1
/
2
[HISA] = ae
+ b
(22)
where a = 4.38 ± 0.07 mM and b = 0.66 ± 0.01 mM. The transformation from
−
ISAL to ISA is more rapid and it is complete in less than 8 min, which is below
the resolution of the ion chromatograph.

Characterization of α-Isosaccharinic Acid: Lactone and Carboxylic Conformations
473
Fig. 4. The evolution of the isosaccharinic containing solutions with
the last obtained pH values in each solution inserted in the figure.
The three end-point titrated solutions are shown with closed circles.
The equilibrium solutions in the pH interval 6–10 fluctuated considerably and
only slowly converged towards the intended pH, as is shown in Fig. 4. Considering
the reactions involved, (1) and (3a), and the fact that the target pH values were
approached from pH 1.2 solutions, it seems plausible that an initial fast transfor-
−
mation to ISA , followed by slow transformation back to ISAL, may be the cause
for this behavior. The transformation reactions led to long end-point titration times,
and these samples were subsequently exposed to air for a considerably longer time
than the other samples with an accompanying carbon dioxide effect. These effects
make it difficult to assess the data points and the associated uncertainties, and they
have subsequently not been used in the equilibrium evaluation. The equilibrium
data can be seen in Table A.II.
Correcting the pKa value previously reported by Ekberg et al.(16) for 22.8 C
◦
and 1 M ionic strength to 0.1 M ionic strength using Eqs. (17)–(19) with ε( j,k,I ) =
m
0.08 (since no specific ion-interaction parameter value could be found for isosac-
charinate, the value 0.08 for acetate was used as an approximation), results in a
pKa value of 3.86 ± 0.05, see Table I. Using this corrected value with Eqs. (2),
(4a), and (6), the equilibrium constant for reaction (3a) can tentatively be deter-
mined to be K1 = 6.88 ± 0.64 by curve fitting the experimental data employing
2
(22)
a χ method to estimate the standard deviation. In Fig. 5, a speciation diagram
based on the these equations and constants is shown.
Spectrophotometric analyses of the lactone and carboxylic conformations
did not result in any useful quantitative information. The overlap of spectra for

474
Ekberg, Ekberg, and Albinsson
Table I. pKa Values Previously Reported in Literature
◦
Temperature ( C)
Ionic strength (M)
pKa
References
Rai et al.(12)
a
2
2
3 ± 2
—
0
4.46
b
c
(13)
2.0
3.27 ± 0.01 /3.36 ± 0.02 Cho et al.
d
(14)
—
—
<0.002
0.0006–0.004
4.05
4.02
Motellier et al.
d
(15)
Motellier and Charles
e
e
e
(16)
2
3
4
2
2.8
1.0
1.0
1.0
0
3.78 ± 0.05
3.64 ± 0.02
3.53 ± 0.03
Ekberg et al.
Ekberg et al.
Ekberg et al.
(16)
4.5
4.2
5
(16)
d
d
(17)
3.87 , 3.98
Hagberg et al.
a
b
c
Fitting of Ca-isosaccharinate solubilities.
NMR, NaISA solution.
NMR, Ca(ISA)2 solution.
Capillary zone electrophoresis.
Potentiometric titration.
d
e
the two conformations as well as the sensitivity to hydroxide concentration in
the studied interval, 180 nm < λ < 500 nm, made analyses very unpredictable
at best. Both conformations showed only negligible absorption in the interval
500 < λ < 2300 nm.
−
Fig. 5. The relative concentrations of α, ISA , HISA, and ISAL are
shown as a function of pH. Measured values of α are plotted with two
standard deviations uncertainty. The three end-point titrated samples
are shown with open symbols.

Characterization of α-Isosaccharinic Acid: Lactone and Carboxylic Conformations
475
5. CONCLUSIONS
The equilibrium constant for the transformation between α-D-isosaccharino-
,4-lactone and α-D-isosaccharinic acid has been established successfully using an
1
ion chromatographic system. The results obtained using both long-term studies of
equilibrium concentrations of the different isosaccharinic species as a function of
pH, as well as evaluation of the transformation rate from α-D-isosaccharinic acid
to α-D-isosaccharino-1,4-lactone are in agreement with one another. On the basis
of the kinetic data, the rate constants for the forward and backward reactions have
also been determined. The transformation from α-D-isosaccharino-1,4-lactone to
isosaccharinate is faster than the resolution of the chromatographic system. In the
assessment of an isosaccharinic containing system, all three conformations should
be considered in order to correctly quantify their concentrations, and to correctly
assess their effects since their individual properties vary considerably. Clearly, this
is of interest when studying, for example, complexation involving isosaccharinic
species, as the complexing properties are attributed to the carboxylic form.
ACKNOWLEDGMENTS
We thank Dr Paul Brown for his valuable input during the writing of this
paper. This work was funded by SKB, the Swedish Nuclear Fuel and Waste
Management Co.
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2
3
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7
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0. G. Getahun, P. Warwick, S. Zauner, and N. Trautmann, Annual Report (Institut f u¨ r Kernchemie,
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1
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APPENDIX
−
Table A.I. Measured Concentrations of [ISA]tot and [ISA ] + [HISA], and [ISAL],
Calculated Using Eq. (6) as a Function of Time in the Transformation Rate Study
−
Time (min)
[ISA]tot (mM)
[ISA ] + [HISA] (mM)
[ISAL] (mM)
0
8
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.06
4.68
4.37
4.16
3.98
3.76
3.63
3.44
3.33
3.16
3.04
2.90
2.76
2.63
2.51
2.42
2.33
2.25
2.12
2.04
1.95
1.88
1.81
1.71
1.68
1.48
1.43
1.32
1.24
1.19
1.14
1.09
−0.02
0.36
0.67
0.88
1.06
1.28
1.41
1.60
1.70
1.87
1.99
2.13
2.28
2.41
2.53
2.62
2.71
2.79
2.92
3.00
3.09
3.16
3.23
3.33
3.36
3.56
3.61
3.72
3.80
3.85
3.90
3.95
1
3
4
5
6
7
8
9
9
0
0
1
2
2
3
4
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
3
4
04
15
25
36
47
57
68
79
89
00
11
21
32
42
53
85
06
27
47
68
89
09

Characterization of α-Isosaccharinic Acid: Lactone and Carboxylic Conformations
477
Table A.I. Continued
−
Time (min)
[ISA]tot (mM)
[ISA ] + [HISA] (mM)
[ISAL] (mM)
4
4
4
4
5
5
5
5
5
8
8
8
8
30
51
71
92
13
33
54
75
95
03
24
45
65
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
1.02
0.96
0.98
0.93
0.87
0.87
0.83
0.83
0.78
0.74
0.71
0.72
0.68
0.68
0.66
4.02
4.07
4.06
4.11
4.17
4.17
4.21
4.21
4.26
4.30
4.33
4.32
4.36
4.36
4.38
1
1
279
290
Table A.II. Final Values of α as Function
of pH in the Equilibrium Study
Sample
pH
α
0
1
2
3
4
5
6
7
8
9
1.15
3.82
4.35
0.13
0.46
0.51
0.55
1.04
1.03
1.03
0.46
0.83
1.02
4.88
10.36
11.03
11.52
4.78
5.29
7.35